The present invention relates to a hydraulic controller used for a variable capacity hydraulic transmission which comprises a hydraulic pump and a hydraulic motor. This pump is driven by an engine to deliver hydraulic oil that actuates the motor, which then generates a rotary driving power .
Various types of hydraulic transmissions have been proposed each comprising a hydraulic pump and a hydraulic motor, at least one of which is in variable capacity. Pumps and motors of swash plate and plunger type are well known for use as hydraulic variable delivery pumps and variable displacement motors in such transmissions. In addition, there is another type of hydraulic transmission which is known as hydromechanical transmission (e.g., Japanese Laid- Open Patent Publication No. S62(1987)- 147148). This type of transmission is designed to increase the efficiency of power transmission and includes a mechanism for carrying out mechanical power transmission, which is provided parallel to a power transmission mechanism that comprises a hydraulic pump and a hydraulic motor. The above mentioned former type, which comprises only a hydraulic pump and a hydraulic motor, is generally referred to as xe2x80x9chydrostatic transmissionxe2x80x9d. However, here, in this document, both types, including the latter, are referred generally as xe2x80x9chydraulic transmission.xe2x80x9d
There is a well known variable speed control which is used, for example, to control a vehicle that comprises such a hydraulic transmission. In this variable speed control, a value which corresponds to the accelerator opening (i.e., the amount of accelerator operation, engine throttle opening, etc.) is defined as xe2x80x9caccelerator indexxe2x80x9d, and a value which corresponds to the vehicle speed or to the rotational speed of the motor that is proportional to the vehicle speed is defined as xe2x80x9cvehicle speed index.xe2x80x9d In relation to these values, a target engine rotational speed is predetermined, and the capacities of the hydraulic pump and the hydraulic motor are controlled to bring the actual engine rotational speed to this target engine rotational speed. To control the capacities of the pump and the motor, the respective swash plates of the pump and the motor are tilted, i.e., the skew angles of the swash plates are controlled.
The control of the skew angles of the swash plates of the pump and the motor is carried out by the expansion and contraction of the servo cylinders which are provided to the pump and the motor, respectively. This expansion and contraction of the servo cylinders is effected by appropriating the line pressure of the hydraulic circuit of the transmission. These pressures which are required to actuate the respective servo cylinders vary in correspondence with the moment forces that act on the swash plates of the pump and the motor, respectively, and they become maximum when these moment forces become maximum. Therefore, for the servo cylinders to be effective all the time, the line pressure must be large enough to meet the maximum moment forces.
However, in practice, the moment forces acting on the swash plates rarely reach the expected maximum values. This means that maintaining the line pressure to such high level all the time so as to meet the requirements of the maximum moment forces, which rarely occur, or keeping the line pressure always above a general or normal requirement is wasteful and not fuel efficient because this condition burdens the engine.
The present invention is to solve the above problem. Therefore, it is an object of the present invention to provide a hydraulic controller which is able to maintain a hydraulic pressure necessary for the control of the skew angles of the swash plates of the pump and the motor that are incorporated in a variable capacity hydraulic transmission.
In order to achieve this objective, the present invention provides a hydraulic controller for a variable capacity hydraulic transmission which comprises a hydraulic pump and a hydraulic motor, the hydraulic pump driven by a prime motor (e.g., the engine E in the following embodiment) delivering oil that actuates the hydraulic motor into rotation. This hydraulic controller comprises hydraulically actuated swash plate tilting means (e.g., the servo cylinders 92 and 96 in the embodiment) which tilts the swash plate of at least either the hydraulic pump or the hydraulic motor (e.g., the pump swash plate 32 and the motor swash plate 38 in the embodiment) around an axis to control the capacity thereof. The hydraulic controller further comprises pressure controlling means (e.g., the regulator valve 60 and the first linear solenoid valve 51 in the embodiment) which adjusts a hydraulic pressure (e.g., the line pressure PL in the embodiment) used for actuating the swash plate tilting means. The hydraulic controller also comprises estimating means (e.g., the control unit ECU and the various sensors in the embodiment) which estimates a moment force that acts on the swash plate from the above mentioned at least one of the hydraulic pump and the hydraulic motor. In this hydraulic controller, the pressure controlling means adjusts the hydraulic pressure in correspondence with the moment force estimated by the estimating means.
In this way, the hydraulic pressure required for the actuation of the swash plate tilting means is prevented from becoming unnecessarily high. Therefore, the load of the prime motor is reduced, thereby improving the fuel efficiency. Hire, the term xe2x80x9cmoment forcexe2x80x9d refers to a periodic force which acts around the trunnion of the respective swash plate while the hydraulic pump or the hydraulic motor rotates with the pistons thereof reciprocating inside the cylinders because of the inside pressures which vary accordingly.
Preferably, the hydraulic controller includes a hydraulic closed circuit which comprises a first oil passage connecting a port of the hydraulic pump (e.g., the port 24b of the hydraulic pump 24 in the embodiment) with a port of the hydraulic motor (e.g., the port 25a of the hydraulic motor 25) and a second oil passage connecting another port of the hydraulic pump (e.g., the port 24a of the hydraulic pump 24 in the embodiment) with another port of the hydraulic motor (e.g., the port 25b of the hydraulic motor 25). In this hydraulic closed circuit, the estimating means comprises first pressure detecting means (e.g., the pressure sensor 208 in the embodiment) which detects the pressure of the first oil passage and second pressure detecting means (e.g., the pressure sensor 209 in the embodiment) which detects the pressure of the second oil passage, and the estimating means estimates the moment force from the pressure which is higher of the two pressures of the first and second oil passages detected by the first and second pressure detecting means. In this way, the adjustment of the hydraulic pressure for the swash plate tilting means is carried out accurately because the varying moment force affects mostly the pressure of the oil passage which is at the higher pressure in the hydraulic closed circuit, and the moment force is estimated from this pressure.
Preferably, the estimating means further comprises skew angle detecting means (e.g., the pump swash plate angle sensor 206 and the motor swash plate angle sensor 207 in the embodiment) which detects the tilted angle of the swash plate (e.g., the skew angle xcex1 of the pump swash plate and the skew angle xcex2 of the motor swash plate) and rotational speed detecting means (e.g., the pump rotational speed sensor 215 and the motor rotational speed sensor 216 in the embodiment) which detects the rotational speed of at least either the hydraulic pump or the hydraulic motor. Thereby, the detected moment force may be corrected on the basis of the type of the valve plate (e.g., the valve plate 150 or 160 in the embodiment) of the above mentioned at least either of the hydraulic pump or the hydraulic motor, of the skew angles of the swash plates detected by the skew angle detecting means, of the rotational speeds detected by the rotational speed detecting means, and of the operational condition of the transmission. In this way, the precision of the above mentioned adjustment of the hydraulic pressure can be improved. The term xe2x80x9coperational condition of the transmissionxe2x80x9d refers to whether the driving force input into the transmission comes through the hydraulic pump (i.e., acceleration) or through the hydraulic motor (i.e., deceleration).
Further scope of applicability of the present invention will become apparent from the detailed description given hereinafter. However, it should be understood that the detailed description and specific examples, while indicating preferred embodiments of the invention, are given by way of illustration only, since various changes and modifications within the spirit and scope of the invention will become apparent to those skilled in the art from this detailed description.